GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2003 🢒 Vibe - FWD 🢒 Engine 🢒 Engine Controls - 1.8L (LNK) 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Rough, Unstable, or Incorrect Idle, Stalling

Checks

Action

DEFINITION: The engine runs unevenly at idle. If severe enough, the engine or vehicle may shake. The engine idle speed may vary in RPM. Either condition may be severe enough to stall the engine. The engine idles at an incorrect speed.

Preliminary Check
    • Refer to Symptoms - Engine Controls .

Sensor Checks
    • Inspect the heated oxygen sensors (HO2S) 1 and HO2S 2 for a condition that can affect sensor activity and response. Refer to Diagnostic Aids for the following DTCs:
       -  DTC P0130
       -  DTC P0133
       -  DTC P0135 or P0141
       -  DTC P0136
    • Inspect the throttle position (TP) sensor. A sticking throttle shaft or a binding throttle linkage will causes a high TP sensor voltage. Under these conditions the powertrain control module (PCM) may not control the idle. Monitor the TP sensor angle with the scan tool. The indicated angle should be approximately 11 percent with the throttle closed.
    • Inspect the engine coolant temperature (ECT) sensor. A fixed or inaccurate ECT sensor reading can cause the engine idle to surge or race. Using the scan tool, compare the ECT with the ambient air temperature on a cold engine. If the engine coolant is 5°C (9°F) more than or 5°C (9°F) less than the ambient air temperature, inspect the resistance of the coolant sensor. Refer to Temperature Versus Resistance . If the ECT is significantly less than the ambient air temperature, inspect the coolant sensor electrical circuit for high resistance.
    • Inspect the mass air flow (MAF) sensor operation. Run the engine to normal operating temperature. Turn OFF all accessories and observe the MAF sensor at idle with a scan tool. If the MAF sensor reading is more than 3.5 g/s refer to DTC P0101 . If the MAF sensor reading is less than 1.0 g/s, inspect for a vacuum leak between the MAF sensor and the engine.

Fuel System Checks
    • Inspect the operation of the fuel system for rich or lean condition. Operate the vehicle under the conditions that caused the concern. Monitor the Fuel Trim parameter on a scan tool in order to identify the problem.
       - Lean--The Long Term Fuel Trim will be more than 20 percent. Refer to Diagnostic Aids in DTC P0171 .
       - Rich--The Long Term Fuel Trim will be less than -20 percent. Refer to Diagnostic Aids in DTC P0172 .
    • Test the fuel pressure while the condition exists. Refer to Fuel System Diagnosis .
    • Inspect the evaporative emission (EVAP) control system. Refer to Electronic Ignition (EI) System Diagnosis .
    • Test the fuel injectors for leaking. Refer to Fuel Injector Balance Test with Special Tool .

Ignition System Checks
    • Test for the proper secondary voltage output with a J 26792 Spark Tester.
    • Inspect the spark plugs for correct operation and good ignition system performance. Refer to Spark Plug Inspection .
    • Inspect the ignition coil boots for damage.
    • Observe the IAC Motor Command parameter on the scan tool at idle, while operating the rear defogger, the A/C system, or the headlights. The IAC Motor Command parameter should increase 2-7 percent when operating the engine loads. If the IAC Motor Command parameter remains fixed, check for a short to ground in the Diagnostic Request circuit from the PCM to terminal 13 of the data link connector (DLC) OBD2. Refer to Engine Controls Schematics .

Engine Mechanical Check
    • Inspect for the following engine mechanical conditions:
       - Any faulty hydraulic lifter assemblies
       - Any broken or weak valve springs
       - A low compression
       - An incorrect valve timing
       - Any sticking or leaking valves
       - An incorrect or worn camshaft
    • Refer to the Diagnostic Starting Point - Engine Mechanical in Engine Mechanical.

Additional Checks
    • Inspect for vacuum leaks. Vacuum leaks can cause a higher than normal idle speed.
    • Verify the PCM grounds are clean, tight, and in the proper locations. Refer to Engine Controls Schematics .
    • Check the scan tool to determine if the PCM is receiving an A/C signal. The idle speed should be increased with the A/C ON. Refer to Electrical/Accessory Load Idle-Up Signal Diagnosis .
    • Inspect the battery cables and the ground straps. They should be clean and secure.
    • Inspect the A/C refrigerant pressure for being too high or for a faulty high pressure switch. Refer to Air Conditioning (A/C) System Performance Test in Heating, Ventilation, and Air Conditioning.
    • Test for the proper operation of the IAC system. Refer to Idle Air Control (IAC) System Diagnosis .
    • The engine will run rough and the engine can stall if the camshaft position (CMP) actuator solenoid is advancing camshaft timing at idle. Inspect for a fully energized or stuck CMP actuator solenoid. If the operation of the CMP actuator solenoid is suspect, refer to DTC P1349 .
    • Inspect the positive crankcase ventilation (PCV) valve for correct operation and the correct part. Refer to Crankcase Ventilation System Inspection/Diagnosis in Engine Mechanical.
    • Inspect that the idle speed is increasing for changes in electrical loads on the engine. Refer to Electrical/Accessory Load Idle-Up Signal Diagnosis .
    • Verify that the PCM is increasing the idle speed when the P/S system pressure is increased on turns. Refer to Electrical/Accessory Load Idle-Up Signal Diagnosis .
    • Inspect for broken or worn motor and transmission mounts.